This invention relates generally to filter press membrane electrolytic cells. More specifically, it relates to the structure and operating conditions which permit a filter press membrane cell to be operated at high current densitites.
Chlorine and caustic, products of the electrolytic process, are basic chemicals which have become large volume commodities in the industrialized world today. The overwhelming amounts of these chemicals are produced electrolytically from aqueous solutions of alkali metal chlorides. Cells which have traditionally produced these chemicals have come to be known as chloralkali cells. The chloralkali cells today are generally of two principal types, the deposited asbestos diaphragm-type electrolytic cell or the flowing mercury cathode-type.
Comparatively recent technological advances, such as the development of dimensionally stable anodes and various electrode coating compositions, have permitted the gap between electrodes to be substantially decreased. This has dramatically increased the energy efficiency during the operation of these energy-intensive units.
The development of a hydraulically impermeable membrane has promoted the advent of filter press membrane chloralkali cells which produce a relatively uncontaminated caustic product. This higher purity product obviates the need for caustic purification and reduces the need for concentration processing. Initially the use of a hydraulically impermeable planar membrane has been most common in bipolar filter press membrane electrolytic cells. Some filter press membrane cells, especially in the bipolar electrode design, have attempted to use a dual cathode surface comprising a first layer of coarse supporting mesh to serve as a current distributor and a finer mesh cathode screen on top of the coarse supporting mesh as the second layer. Other cell designs have recognized the need for obtaining uniform current distribution, especially in cells of a monopolar design, but have failed to achieve this for several reasons, for example because of the use in wide, short cells of a bus bar carrying current across the width of a cell, but near the cell bottom, so that the electrode material has to carry the current vertically upwardly in the cell. However, continual advances have been made in the development of monopolar filter press membrane cells.
Despite these continued advances in the filter press cell technology, the high initial capital cost to build a electrolytic cell facility has deterred large scale construction of these type of cells in the industry. Attempts to reduce these high capital costs have recently focused on the ability to operate the cells at elevated current densities to permit fewer cells to be able to produce more product than is conventionally produced at lower current densities in the two to three kiloampere per square meter range. However, such attempts have met with problems because of the heat buildup within the operating cell. This heat buildup results from the resistance that the cell components generate to current flow through the cell. The cell has metal parts such as conductor rods, electrode frames, bus bars, the cathodes and the anodes that contribute to the voltage coefficient resistance, which is the sum of the resistances of the cell components, the membranes and the electrolyte to current flow. Filter press membrane cells, in the past, have had typical hardware or cell component resistances of approximately 250 millivolts at current densities in the 3 kiloampere per square meter range.
As the heat builds within the cell, the electrolyte temperature increases and can even reach the boiling point. This elevated temperature can cause the water to be removed from the cell, such as by evaporation or boiling off, especially in the anolyte, faster than it is replaced. The permselective ion exchange membranes are also affected by this elevated temperature. The polymer chains on current membranes can delaminate from each other because of elevated operating temperatures, which will cause blisters in the membrane. The membranes also can rupture or burst due to the water boiling within the membrane because of the heat generated by the electrical resistance within the membrane. In order for the membrane to function properly, the water must remain in the liquid phase. The elevated temperature and the boiling of the water can cause the membranes to delaminate when a cell is operated at a current density above 4.0 kiloamperes per square meter over a period as short as a few minutes, depending upon cell size.
These problems are solved in the design of the present invention by controlling the voltage coefficient or summation of resistances of the cell, expressed in terms of the current density, below a predetermined level to obtain a heat and material balance which, because of the lower cell resistance, permits the cell to be operated at higher current densities.